Field of the Invention
The present invention relates to the technology of stretching material sheets or films, particularly stretching polymer material sheets or films, on stretching plants. Particularly, the present invention relates to the field of processes of stretching material sheets or films, particularly polymer material sheets or films, on such stretching plants in the transverse direction, and also relates to devices improving the transverse stretching of polymer material sheets or films, which devices form part of the overall stretching plant.
Discussion of Background Information
The first step of an overall process of fabricating oriented films comprising polymer materials is an extrusion, through a die, of a film of a molten polymer onto a roll (“chill roll”), the surface of which is kept at a temperature below the glass transition temperature of the respective polymer. The roll surface quenches the polymer into an amorphous state.
A subsequent step of said process comprises steps of stretching the film previously generated by well-known methods in the longitudinal (machine) direction and/or in the transverse direction, the latter being a stretching step in a direction perpendicular to the machine direction, commonly. These stretching steps may be performed one after the other or may be performed simultaneously, or even only one stretching step may be performed, e. g. in the transverse direction.
In the case of sequential stretching, the stretching process itself is carried out by two machine units, usually first in the longitudinal/machine direction (i. e. in the direction of the travelling path of the polymer film on the stretching device) and then in the transverse direction. In the case of simultaneous stretching, a stretching action in the longitudinal (machine) direction and transverse direction needs the simultaneous application of stretching forces to the film material in two directions (usually perpendicular to each other), while the film is moving with high speed along a moving path of the film on a stretching plant or stretching device. The polymer film is moved along a longitudinal path on said stretching plant or stretching device by carriages, a large number of which are fixed to, and drawn by, endless chains to run on rails on both sides of said longitudinal path either on wheels or on gliding members between the carriages and the rails. Two opposite lateral edges of the polymer film are fixed to said two opposite carriages on tenter tables by means of clamps, said carriages stretching the polymer film in the longitudinal direction and—as the case may be—stretching the film also—or exclusively—in the transverse direction.
Once the drawing step(s) is/are completed, the drawn polymer film is “heat set” or crystallized under tension and with a temperature gradient starting at elevated temperatures and continuing to decreased temperatures. The heat setting step prevents the film from shrinking back to its unstretched shape and “locks” the molecular orientation in the polymer film plane. The orientation of the polymer molecule chains thus obtained is responsible for the high strength and stiffness of the oriented film.
Together with the molecular orientation, the formation of polymer crystal nuclei is induced. The crystallites grow rapidly and reach the boundary of neighbouring crystallites. Their size is smaller than the wavelength of the visible light. As a result, oriented polymer films have an excellent clarity, despite the semi-crystalline structure of the polymer units. Particularly, polymer films obtained by simultaneous biaxial stretching have excellent mechanical and optical properties and, hence, are highly valued.
At the end of the stretching and heat-setting areas of the stretching plant or stretching device, the clamps fixing both lateral edges of the polymer film on the tenter tables of the carriages release the film from the tenter tables, and the film produced and stretched is coiled on a film storage roll or is further processed, while the carriages with their clamps released from the tenter tables return to the starting point of the stretching process for another stretching cycle.
Presently, stretching plants or stretching devices for transversely stretching polymer films are in operation. Such plants or devices are described in the document WO-A 2004/108,395, the entire disclosure of which is incorporated by reference herein, which relates to polymer sheet stretching systems using a series of gripping means for the support, transport and drawing of a polymer film to be stretched.
Such gripping means are supported and guided by rails and driven by endless chains. Each chain is driven by two toothed wheels of large diameter, one wheel being located at the beginning of the forward run and the other wheel being located at the rear end of the return run of the chain. Each wheel is provided with motorised rotational drive means, said drive means being synchronised with each other. Such a drive device finds application in high speed drawing machines.
These stretching plants or stretching devices of the prior art have the drawbacks that, particularly in the case where the carriages are driven by the endless chains on wheels moving on the rails 60 (see FIG. 1), there is only one long tenter table with its clip fixing the polymer web per every two chain links. Hence, the tenter table is fixed to one chain link and extends almost over the whole next chain link. Herein the term “chain link” is used in the present claims and specification to ads dress the outer plates (“outer chain links 51”) of the endless chains 50 and inner plates (“inner chain links 52”) connecting the centers of two consecutive pins 53 defining the chain links' rotational axis, as is exemplarily shown in FIGS. 1A and 1B. In FIG. 1B, the chain pitch, i.e. the distance between the centers of two consecutive pins 53 defining the chain links' rotational axis, is indicated to be “P”. The one long tenter table extending over almost two chain links, having a length of slightly shorter than 2 P (wherein “P” is the pitch as defined above) and fixing the polymer web by means of the one clip creates a certain rigidity of the polymer web fixation, resulting often in wrinkles of the polymer web generated in the process of stretchingly moving the web. Such wrinkles deteriorate the stretched polymer web's quality and, hence, are not desired.
It was surprisingly found that the drawbacks of the prior art can be overcome by providing dual tenter clips and connected tenter tables on a wheel-guided clip carriage assembly comprising a main clip body and a secondary clip body. Thereby, the table length could be reduced, preferably to almost 50% of the length of tenter tables of the prior art. Moreover, a greater flexibility of the polymer web movement could be achieved for the stretching step, accompanied by a safer fixation of the moving polymer web to the respective shorter tenter table for the stretching step(s). Hence, the appearance of wrinkles could be reduced and even prevented completely.